The present invention relates to systems for controlling the application of power to alternating current electric motors; and in particular to such devices which provide a controlled starting and stopping of the motor.
A conventional motor controller has thyristors which connect the motor windings to alternating current supply lines. For a three-phase motor, each AC phase line usually is coupled to a separate winding within the motor by either a triac or a pair of inversely connected silicon controller rectifiers (SCR's). A circuit within the controller determines the proper time to trigger the thyristors during each half-cycle of the supply line voltage. The thyristors are triggered in sequence as determined by the phase relationship of the voltage on each supply line. The sequence is circular in that after each iteration of triggering all of the thyristors, the process repeats by re-triggering the SCR's in the same order. Once a thyristor is triggered it remains in a conductive state until the alternating current flowing therethrough makes a zero crossing at which time it must be retriggered to remain conductive. By regulating the trigger times of the thyristors with respect to the zero current crossings, the intervals during which they are conductive can be varied to control the amount of voltage applied to the motor.
To start the motor, conventional motor controllers vary the thyristor trigger times to provide a gradual increase in the voltage. In doing so, the thyristors are initially triggered relatively late in each voltage half-cycle so that they are conductive for only a short period. The trigger times then become progressively earlier in each half-cycle to render the thyristors conductive for longer intervals and apply greater amounts of voltage to the motor.
As the thyristors become triggered earlier in each half-cycle during starting, the motor speed increases until it reaches full speed. Many previous motor controllers continued to utilize the SCR's to apply electricity during full speed operation, in which the SCR's essentially are always in a conductive state. When the motor is at full speed, the SCR's produce a significant amount of heat, which must be dissipated. Therefore, for continuous operation a considerably larger heat sink is required and forced ventilation may be necessary so that excessive heat does not build up in the controller's enclosure.
To minimize the size of the heat sink and cooling requirements, hybrid solid state controllers and contactor systems were devised. One such system is described in U.S. Pat. No. 4,100,469. That system has a acceleration circuit which started the motor by regulating the triggering of SCR's in the previously described manner. However when the motor reaches full synchronous speed, a set of contactors in parallel with the SCR's closes and the SCR triggering is discontinued. Therefore a full speed the motor current flows through the contactors, not the SCR's and the SCR's do not generate heat which must be dissipated.
More recently solid state motor controllers have been employed to provide a maneuver to stop the motor. For example, an SCR triggering method inverse to that used to start the motor can be used to stop it. That is, the SCR's are triggered successively later during each voltage half-cycle to apply progressively smaller amounts of power to the motor. Alternatively, a cycle-skipping technique can be employed to reduce the motor speed. However, these deceleration techniques previously could only be employed readily with controllers which continued to trigger the SCR's during full speed operation. In the hybrid system, once the contractor closed and the SCR triggering ceased, the control circuit lost synchronization with the AC supply line voltage. As a result, the control circuit could not be used to smoothly brake the motor since it did not know where to begin the pattern of SCR triggering at the start of the deceleration. Unless the controller knows at which point of the sequence to begin triggering the SCR's, a smooth transition to SCR control can not occur and the motor may abruptly jerk the equipment driven by it.